In recent years, display devices that include active matrix substrates have become widely used. The active matrix substrate includes, for each pixel, a switching element such as a thin film transistor (referred to as a TFT below). An active matrix substrate including TFTs as switching elements is referred to as a TFT substrate.
A TFT substrate used in a liquid crystal display device or the like includes a glass substrate, a plurality of TFTs supported by the glass substrate, gate wiring, source wiring, and pixel electrodes arranged in a matrix, for example. A gate electrode, a source electrode, and a drain electrode in each of the TFTs are electrically connected to the gate wiring, the source wiring, and the pixel electrode, respectively. The TFT, the source wiring, and the gate wiring are normally covered by an interlayer insulating layer, and the pixel electrode is provided on the interlayer insulating layer and connected to the drain electrode of the TFT inside a contact hole formed in the interlayer insulating layer.
Conventionally, TFTs having an amorphous silicon film as the active layer (hereafter “amorphous silicon TFTs”) or TFTs having a polycrystalline film as the active layer (hereinafter “polycrystalline silicon TFTs”) are widely used. Recently, oxide semiconductors have been given attention as a material for the active layer of TFTs. The oxide semiconductors have a higher mobility than amorphous silicon. Therefore, the oxide semiconductor TFTs can operate at a faster speed than the amorphous silicon TFTs. Because an oxide semiconductor film can be formed by a simpler process than a polycrystalline silicon film, the oxide semiconductor film can be employed in a device requiring a large surface area. In the present specification, a TFT having an oxide semiconductor film as the active layer may be referred to as an “oxide semiconductor TFT.”
The source electrode and the drain electrode of a TFT are generally formed from the same conductive film from which the source wiring is formed. Aluminum (Al) or an Al alloy having high conductivity is widely used for the material for the conductive film. Recently, using copper (Cu), which has even higher conductivity, has also been proposed. In the present specification, a layer formed from the same conductive film including the source wring may be referred to as the “source wiring layer.”
By forming the source wiring layer using Al or the like described above, source wiring with small wiring resistance can be formed. On the other hand, when the Al film (or Al alloy film) makes contact with the semiconductor layer of the TFT, there is a concern that Al will diffuse into the semiconductor layer and that the desired TFT characteristics become unobtainable. Similarly, when forming a source wiring layer using Cu, there is a concern that Cu will diffuse into the semiconductor layer and that the desired TFT characteristics become unobtainable. Also, in a manufacturing process of a TFT substrate, when performing heat treatment (approximately 200 to 600° C., for example) after forming the source wiring layer, the surface of the Al layer can be modified because of the Al layer (or Al alloy layer) being heated, and a protrusion called a hillock can be generated. A hillock on the surface of the Al layer reduces the insulation characteristics of the interlayer insulating layer.
On the basis of these considerations, using a multilayer film to form a source wiring layer has been proposed. Patent Documents 1 and 2 disclose a source electrode and a drain electrode having a structure in which a molybdenum (Mo) layer, an Al layer, and a Mo layer are sequentially layered, for example. Patent Document 3 discloses a configuration in which a titanium (Ti) layer is formed between an Al layer and an oxide semiconductor layer or between a Cu layer and the oxide semiconductor layer. Patent Document 4 discloses a source electrode and a drain electrode having a structure in which a Ti layer, an Al layer, and a Ti layer are sequentially layered.